Semiconductor technology today is highly sophisticated, and in keeping with this high sophistication there is a requirement for rapid diagnostic techniques. In order to optimize a process to obtain best performance from a device (or circuit), it is imperative that diagnostic techniques used be rapid and require little, if any, extraneous processing which can confuse the relevance of the measurement results. Minimal delay between a processing or design change and the measured result of that change is desired so that the information feedback loop can be as rapid as possible.
Minority carrier lifetime is a physical parameter of interest to almost every semiconductor device and materials specialist who wishes to evaluate material quality. A host of techniques is utilized for lifetime measurements at the various stages of semiconductor processing. Some of the more predominant techniques utilized are photoconductive decay, MOS capacitive decay, diode saturation current, diode open circuit voltage decay and SPV (Surface Photo-Voltage) measurements.
Each of these techniques suffers in some way which limits its applicability, and in addition often gives results which can be ambiguous. Photoconductive decay techniques are attractive since little processing is required, although some form of electrical contact is necessary. Unless background lighting is used to fill traps, measurement results can be difficult to interpret. Photoconductive decay is hard to apply to wafers at various stages of processing because of the difficulty of probe contact in anything but coarse geometries. Also, the nature of the contact (Schottky) can influence the results. According to the present invention, an optical beam of sufficient energy to excite free carriers impinges on a localized region of a semiconductor sample. Amplitude modulation of the optical beam results in time-dependent fluctuations of the free carrier density with a phase lag which increases with the free carrier lifetime. Another optical beam with an energy insufficient to excite free carriers but whose absorption depends on the free carrier density passes through the sample including the localized region. This second optical beam is thus amplitude modulated in phase with the free carrier fluctuation; phase comparison of the modulation on both beams is used to calculate the sample lifetime. The accordance with the disclosed invention has the following benefits.
The MOS and diode techniques suffer from requiring processing to be performed on the material. High temperature processing is usually required for the diode techniques, and unless this can be part of a normal processing step, this processing can totally alter the relevance of the measured lifetime. The MOS technique can be performed at lower temperatures if a reasonable electrical contact to the substrate is available. Interpretation of any of these techniques must be carefully done to eliminate surface recombination effects on MOS measurements or junction profile effects on diode measurements.
The SPV technique is distinct in that it actually measures the diffusion length most directly, and indirectly measures the lifetime. In many cases of interest, e.g., solar cells, the diffusion length is the more relevant parameter. This technique however, is indirect in that the absorption coefficient needs to be known accurately, and the degree of strain in the material can alter the absorption coefficients dramatically. In addition, some processing of the wafer is often necessary, and finally, the resolution of the techniques is not good.
All of the above-mentioned techniques have failed to satisfy the need for a fast, reliable technique for lifetime measurement. A new technique is needed and, furthermore, it would seem that only optical techniques can provide a completely in-line, rapid, technique for lifetime measurements.
Lifetime measurements are done optically requiring no contacts or mechanical probing and little, if any, processing is required. Measurements may be made at various processing stages as long as no metalization is present to block the light. Moderate resolution is feasible, e.g., spot sizes of less than 1 mil might be analyzed, depending on various considerations such as optical performance. Very high speed measurements are possible, allowing wafer scanning and tabulation via a data acquisition system.